Need help making an even slope. I'm new
With Pivot Center set to 'Active'..


in Edit>Face mode, select the face you want to flatten to
hit CtrlAltSpace to create a Custom Transform Orientation from the face. (Check 'Use After Creation' and, if you don't want your custom orientations to hang around in a list: 'Overwrite Previous')
in Edit>Edge mode, select the edge-loop around the hole
in Edit>Vertex mode (the hole edge-loop should still be selected, as vertices), also make one of the vertices on the orientation-face active, by Shift-selecting it last.
hit SZZ0, to scale the all your selected vertices to 0 along the normal (the local Z-axis) of the face defining your custom orientation, centered on the active vertex.